Re: Assumptions about the number of parallel workers

Hi,

On 2020-02-07 09:44:34 +0100, Antonin Houska wrote:
> Those Gather nodes still have non-zero num_workers, see this part of
> standard_planner:
>
> if (force_parallel_mode != FORCE_PARALLEL_OFF && top_plan->parallel_safe)
> {
> ...
> gather->num_workers = 1;
> gather->single_copy = true;

Ick. Looks like you might be right...

> Also, if it num_workers was zero for any reason, my patch would probably make
> regression tests fail. However I haven't noticed any assertion failure.

That however, is not at all guaranteed. The regression tests don't run
(or at least not much) with force_parallel_mode set. You can try
yourself though, with something like

PGOPTIONS='-c force_parallel_mode=regress' make check

Greetings,

Andres Freund